As wireless communications has become prevalent in society, it is not uncommon for two wireless communications devices to interfere with the operation of each other when operating within transmission range of each other. Indeed, this type of interference has become more frequent with increasing wireless connectivity permeating society, such as the use of cellular phones and mobile email devices. For example, when IEEE 802.11b (WiFi) wireless devices, for example, laptop computers, became readily available to the home consumer, “cordless” home telephones were capable of rendering the WiFi devices inoperable due to interference.
Interference is typically categorized into two types: narrowband interference and wideband interference. Narrowband interference includes forms of interference that are concentrated around a relatively small frequency bandwidth. Several common approaches to compensating for narrowband interference may include, for example, forward error correction (FEC), baseband coding (FHSS/DSSS spread spectrum), diversity in frequency and/or time, and signal cancellation.
Differently, wideband interference includes forms of interference that are spread out across a relatively large frequency bandwidth. Several common approaches to compensating for wideband interference may include, for example, FEC, adaptive data rate (modulation and channel bandwidth), adaptive power control, diversity in frequency and/or time, and signal cancellation.
Another common type of interference is self interference. Common causes of self interference may include, for example, collocation or frequency reuse. In particular, collocation may be caused by two wireless communication systems being deployed too closely together while frequency reuse may be caused by two wireless communication systems using the same frequency band/channel. Several approaches to compensating for self interference, which are available during planning and development stages of wireless communication systems, include, for example, installation (isolation between antennas), frequency planning, antenna pattern manipulation, synchronization of transmitters, and filtering (duplexing).
In particular, one filtering approach to narrow band interference may be to apply a notch filter to the received signal. The notch filter is a type of band-stop filter that includes a narrow stop band. In typical applications, when a communication system is subject to narrow-band interference, the stop band of the notch filter is tuned to the frequency of the narrow-band interference, thereby mitigating the effect of the narrow-band interference.
In some applications, it may be desirable to mitigate narrow-band interference from a relatively high-power interference source, for example, a jammer device. Typical RF notch filters may be applied in these applications, but they may suffer damage from the high-power interference source. One approach to this drawback may be a photonic implementation of an RF notch filter.
For example, U.S. Patent Application Publication No. 2010/0046952 to DeSalvo et al., also assigned to the present application's assignee, discloses an RF notch filter including photonic components. In particular, this RF notch filter comprises a modulator for modulating an optical carrier signal, and a pair of optical fibers receiving the modulated optical signal. Phase delay is introduced into one of the optical fibers by “lengthening” the optical fiber. The outputs of the optical fibers may be combined to notch filter an input signal via superposition. A potential drawback to this approach may be that this lengthening of one optical fiber may be cumbersome to implement on-the-fly. A potential drawback to typical optical notch filters may be sensitivity to polarization changes due to environmental conditions.
In particular, for the typical optical notch filter that uses a coherent optical source, the optical source may need to have a high degree of coherence. Moreover, the optical phase may need precise control mechanisms, which may add to the cost of the device. Also, coherent applications may be highly sensitive to the aforementioned polarization issue.